1. Field of the Invention
This invention relates to a flexible pipe with improved thermal protection and, more particularly, to flexible pipes in which a fluid flows under high pressure, such as e.g. those used in shore-based and offshore installations for exploration and production of hydrocarbon deposits.
2. Description of the Prior Art
When such flexible pipes are installed in high fire risk areas, such as drilling platforms, or when they are connected to production, linking, control and/or safety means of parts of oil installations, deterioration due to sudden and/or sustained increase in ambient temperature can lead to very serious accidents for both staff and the other parts of the installation in the vicinity of the deteriorated flexible pipes.
Firstly, it should be noted that the diameters of flexible pipes concerned by this invention can vary between approximately 2.5 and 30 cm, and that the pressure circulating inside the flexible pipes can reach 1,000 bars and more.
Under these conditions, any leakage whatsoever of these flexible pipes, especially due to local outbreak of fire, can understandably entail severe consequences. It is for this reason that efforts have always been made to preserve the the event of a fire, in order to enable the rate of flow of the fluid to be maintained without leakage at working pressure until flow stoppage operations have been carried out, or for as long as it takes to bring the fire under control.
Though there are no specific standards in existence, it is recommended to proceed in such a way that the flexible pipes resist for 30 minutes when subjected to an ambient temperature of 700.degree. C.
The flexible pipes used in the above-mentioned applications, which are not in any way restrictive, generally comprise at least one tight sheath and at least one armour layer. The flexible pipes in question typically include at least one layer of reinforcement comprising metal wires, where the cross-section of the wires may take any shape. Flexible pipes often comprise two sheaths, an external sheath surrounding the reinforcement windings, and the other sheath being inside said armour layer, depending on the specific utilizations of said flexible pipes. Certain flexible pipes also comprise a third, intermediate, sheath disposed between two armour layers.
Furthermore, the flexible pipes in question are commonly fitted with an outer protection consisting of a metal strip wound into a low-pitch helix; the strip is e.g. comprised of an interlocking profiled strip. However, it is important to note that this strip was not at all for thermal protection purposes but was simply intended to protect the flexible pipe against surrounding mechanical attacks such as friction and/or crushing. Under these conditions, when the flexible pipe is subjected to the heat from a fire, the plastic sheath(s) have been observed to begin to melt. It is the external sheath that melts first, and then, due to heat conduction phenomena between the elements constituting the flexible pipe, since the heat conductivity of the metal wire windings is very high, it is the internal sheath which gradually softens. When it softens, the internal sheath, which is designed to act as a tightness barrier, is penetrated by extrusion, under the effects of the internal pressure of the fluid, through gaps in the reinforcement, to finally create a leak and enable the fluid to gush out of the pipe.
In the case of thermoplastic materials usually used to manufacture external and internal sheaths, the melting point is between 130.degree. C. and 200.degree. C. Admittedly, the metal strip delays the destruction and/or softening of the sheaths, but cannot prevent this occurring when the temperature is high.
In UK patent application No. 2,180,032, there is described a steel pipe fitted with a thermally insulating layer composed of a relatively thick layer of a mixture of cork and rubber granules. The steel pipe is intended for immersion in sea water and the purpose of the thermal layer is to reduce thermal dissipation towards peripheral layers of the fluid flowing in the steel pipe, in order to obtain, at exit of the steel pipe, a fluid at a suitable temperature.
This invention aims to improve the protection of flexible pipes in or near an area with a prevailingly high temperature liable to modify the mechanical and/or physical properties of at least certain component elements of said flexible pipes.